Coherent optical communications may require optical mixers capable of mixing signal light with local oscillator (LO) light at one or more specific phase shifts. Quadrature phase shift keyed (QPSK) modulation, which provides a high spectral efficiency and good receiver sensitivity in multilevel coherent transmission systems, typically require a 90° optical hybrid (OH) as such optical mixer at the receiver. A 90 degree optical hybrid is a 6-port device with two input and four output ports, which is represented in FIG. 1A at 10. Ports 11 and 12 are input ports for receiving the signal light “S” and the LO light “LO”, respectively, and ports 131 to 134 are the four output ports. The optical hybrid 10 is configured so that when the input signal light “S” and the LO light ‘LO” are received at the input ports 11 and 12, each of the four output ports 13 output a mixture of the signal light and the LO light with the optical phase difference ΔΦS-LO therebetween incremented by 90° from output port to output port (note that the order in which these phase shifts appear in the output ports of an actual device may vary depending on the hybrid's design). Using two pairs of balanced photodetectors (PD) enables then to demodulate the two quadrature components of the optical QPSK modulation in the received signal light “S”.
Performance of an optical hybrid in communication system may depend on phase errors within the device, its insertion loss, power balance between the four output ports, and wavelength bandwidth. The last parameter measures the range of wavelength in which the phase error, insertion loss, and power balance remains within allowed tolerances. The phase error is understood as the deviation of the inter-port phase shift increment ΔΦS-LO from 90°. For long-haul transmissions, the phase error, insertion loss, and power balance metrics should be within design tolerances across the entire C-band.
An optical hybrid may be implemented in a chip as a photonic integrated circuit (PIC) device, for example in the form of a 2×4 or 4×4 multi-mode interference (MMI) coupler, or as a network of 2×2 couplers and 1×2 couplers. In the latter case thermal phase tuning between the couplers is typically used to provide the required 90° phase shifts between ports. A 4×4 MMI coupler is a passive device where the 90° phase shifts are ideally provided by the device geometry without thermal tuning. However, compared to 2×2 couplers, it's more difficult to reach the required phase error and branch power balance tolerances in a 4×4 MMI coupler. For example, a typical phase error in a typical 4×4 MMI coupler may be as high as +/−5 degree. The insertion loss is another concern in designing a 4×4 MMI coupler.
On the other hand, a 2×2 coupler is relatively easier to design to desired tolerances, and typically has a smaller build-in phase error and lower insertion loss than a 4×4 coupler; therefor an optical hybrid built of 2×2 and 1×2 couplers may sometimes be preferable. One drawback of such configurations is that they typically require waveguide crossings to construct the optical hybrid. This would increase the length of total routing waveguide by a substantial amount, introducing phase uncertainties that may eventually degrade the system performance. As a result, thermal phase tuners may be required between the 2×2 and/or 1×2 couplers to control the phase shifts between outputs of different couplers. This thermal tuning complicates the device as it introduced the need to an extra control algorithm, extra optical loss and extra power consumption. It also increases the device footprint as well as complexity of system layout. Additionally, the waveguide crossing may introduce crosstalk to receiver channels.